The use of fixation devices for the treatment of degenerative disc disease, vertebrae and spinal deformities and trauma is well known in the art. U.S. Pat. No. 6,966,910 to Ritland and U.S. Patent Application Publication No. 20050177157 of Jahng thoroughly describe the state of the art, the disclosures of which are incorporated by reference herein. In general, spinal column diseases often require surgical treatment when more conservative management and treatment is unsuccessful. Such surgical treatments are often directed to reducing pressure on nerve roots in the spinal canal or to addressing instability in the spine. Reducing pressure on nerve roots in the spinal canal typically involves spinal decompression which involves the removal of certain tissue from the spinal column. This removal of tissue weakens the spinal column.
Spinal fusion is sometimes used to provide stability to the spine. Spinal fusion is the technique of combining two or more vertebrae using supplementary bone tissue and the human body's natural osteoblastic processes. Spinal fusion prevents relative movement between the fused vertebrae to alleviate pain and injury associated with such movement.
In some cases, spinal fixation is used in conjunction with spinal fusion due to the time which is sometimes required for spinal fusion to provide adequate stability. Spinal fixation refers to the use of rods or plates attached to selected vertebrae by fixing screws so as to hold that portion of the spine in a relatively fixed position. The devices used, i.e. the rods, plates, and screws, are eventually removed from the patient once an adequate amount of spine healing and stability is achieved. Traditional spinal fixation is often referred to as “rigid fixation” because there is little to no movement of the relevant portion of the spine while the fixation devices are in place.
As is also well known in the art, there are serious drawbacks to spinal fusion and spinal fixation techniques. Most prominently, when a small part of the spine is immobilized, the movement of other adjacent parts of the spine is increased in response to the patient's normal movements. The increased movement by parts of the spine adjacent to immobilized vertebrae causes significant strain and wear on those adjacent parts. Such strain could lead to further injury and pain.
As a result, other techniques are sometimes employed for supporting the spine. Dynamic fixation refers to the use of spinal fixation devices which are not completely rigid but which have a selected amount of flex to allow an appropriate amount of movement of the relevant portions of the spine. Such movement has been determined to be beneficial in some cases in aiding a patient's recovery from spinal injury and/or surgery. Many dynamic fixation devices and techniques are disclosed in the prior art.
One type of dynamic fixation system involves using flexible members secured by bone anchors which are attached to a patient's vertebrae. For example, U.S. Pat. No. 6,966,910 to Ritland discloses a dynamic fixation device and a method for its use. The device comprises a flexible rod portion attached to pedicle screws, where the screws are inserted into the vertebrae of the spine. The flexible rod portion is made of elastomeric material or an alloy appropriately shaped to function as a spring and/or pivot. The device allows for limited motion which models the bending motion of the spine.
U.S. Patent Application Publication No. 2005/0177157 of Jahng discloses a method and apparatus for flexible fixation of a spine. The apparatus comprises at least one flexible fixation rod secured to a patient's spinal pedicles by screw-type members. A flexible wire is sometime used, which traverses an axial cavity of the rod members, to connect multiple rod members together. The screw-type members use a cylindrical head, a screw-type shaft, and an outside threaded nut to clamp onto the rod members and screw into the patient's bone tissue.
Many other devices have been used to achieve spine stabilization, whether fixed or dynamic. Unfortunately, these devices have serious drawbacks. One serious drawback is that it is often difficult to install the rod members in the bone anchors. The bone anchors often require that the rod member be installed with a particular orientation relative to the bone anchor. When the rod member is to be installed in more than one anchor, it can be difficult to orient the rod properly without damaging the rod, the bone anchors, or even the body tissues of the patient. The prior art fails to provide pedicle screw to rod connectors that can be easily adjusted at the time of implantation for off-axis, out of plane and non-linear alignment conditions. First positioning the bone screws in the vertebrae for proper holding strength and then trying to position the vertebrae and fit a straight rod or actually trying to bend a rod into the resulting multi-axial geometry at each rod screw head interface or coupling point is a daunting task.
There have also been many attempts to alleviate this difficulty. Most prominently, screws with multi-axial or poly-axial heads are used as bone anchors. A multi-axial head is designed to receive a rod member oriented at one of many different angles relative to the screw. Such a screw is disclosed in, for example, U.S. Pat. No. 5,797,911 to Sherman et. al and U.S. Patent Application Publication No. 2002/0026193 of Barker et al.
The use of pedicle screws with multi-axial or articulating heads to receive the connecting rods does alleviate some of the problems and difficulties in accurately bending the rods to fit the geometry but does not completely solve the problem. First, they are relatively difficult and time-consuming to properly install and adjust. Each screw is generally comprised of multiple components, each of which must be carefully set into the desired position. This further complicates the actual connection of the rod to the screw head in the adjacent vertebrae. It can also cause further difficulties in making final adjustments to the relative position of vertebrae and the interbody space.
Articulating multi-axis screw heads are still cumbersome and alignment results can be poor. Potential damage to the screw and the rod can occur as a result of bending the components in place and this damage could cause premature failure of the fixation system or cause trauma to the vertebrae itself.
What is needed then is a spine stabilization system which is easy to properly install while still providing effective stabilization. The amount of flexibility in the system should be adjustable to allow for use with spine problems of various types. The system should be capable of fixed stabilization as well as varying degrees of dynamic stabilization. The system should also be simple and inexpensive to manufacture.